1. Field of the Invention
The present invention relates to a control system for automatically preventing the shifting of the drive of a two speed rear axle in a vehicle from a low speed mode of operation into a high speed mode of operation when a front axle is drivingly coupled to the engine power train for the vehicle.
2. Description of the Prior Art
Present day 4.times.4 vehicles, namely commercial vehicles, such as utility company work trucks, snow plows, trucks, etc. having single speed front and rear axles are geared to produce axle ratios having a top geared speed of approximately 52 miles per hour (mph) particularly when powered by low speed engines such as diesels. This 52 mph speed is below the present norm of 55 mph highway speed. Further, the single speed axles, while providing the ability of attaining 52 mph highway speeds, do not provide any gradeability to the drive axles for off-road, low gear performance.
Accordingly, it is desirable to provide a vehicle which can reach and maintain an acceptable driving speed during highway travel and yet be capable of being downshifted to a low gear 4.times.4 operation for off-road use.
Further, it is desirable to provide a control system for preventing the front axle from being powered while enabling the vehicle to be operable in a high speed mode at highway speeds without the front axle being powered with the engine power train to protect the transmission and front end drive train from harm caused by high speed driving and axle speed mismatch if the front axle was engaged.
As will be described in greater detail hereinafter, the control system of the present invention includes an electrical control circuit which facilitates an acceptable gear-down gradeability for a vehicle for off-road low speed operation thereof and yet permits high speed operation at acceptable road speeds while preventing high speed operation when the front axle is drivingly coupled to the engine power train.